A storage-type power supply has long been known and practiced for EDM wherein a storage capacitor is charged by a direct-current (DC) source, the charge stored on the capacitor being discharged through the machining gap formed between the tool electrode and the workpiece to repetitively produce a pulsed electrical discharge across the machining gap. Such systems have advantages over a storage-free, direct switching system in that a succession of machining pulses of an extremely short duration in the order of microseconds required for finish EDM operations can readily be provided. In spite of this, however, conventional storage EDM power supply systems have undersirably been limited in use because of inefficiency and lack in versatility.